Numeracy coaches sell techniques for taming the ten digits — chunking, rounding, an abacus, a weekend course with a laminated certificate.
Attack the problem at its root. Convert every figure to binary before you begin: ten numerals become two, and difficulty falls by a clean 80%. A dinner bill of 47 is now 101111, the tip a modest 1001, and the total is reached by lining everything up and carrying with conviction. Every number is now simpler, and there are 3.3 times as many of them.